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Has anyone tried a bolt on fingerboard extension? Not wanting to use this method exclusively, but would like to try it. Looking for ideas, pros and cons.

On what kind of guitar?

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I've done it on 3 guitars. I used two 6-32 T-nuts buried in the fingerboard and top it with a layer of .060" carbon fiber plate that is inlayed flush with the underneath side of the fretboard. I was a little worried about the strength and stability but the guitars have held up well for about 5 years now. The advantage of my design is that I do not have to have a recess routed into the top of the guitar.

The best design I have seen for this is Trevor Gore's bolt on, bolt off neck.

I've been doing it on all of my guitars for a few years. It makes the guitar super easy to work on. I have noticed a little bit more movement/settling of the neck compared to a glued extension.

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I have used the Bourgeois type design with some modifications since 2005. Yes that design does move a little more than a bolt on neck with a glued extension.
You may have to make some modifications to strengthen your upper bout to minimize that movement.

I am good with what I have evolved into but if I was starting from scratch I would take a hard look at Trevor’s system.

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It's not what you don't know that hurts you, it's what you do know that's wrong.

Have you checked if there are indentations in the soundboard binding under the heel? That seems to be the main mechanism of settling without a glued extension. Harder binding material may be able to prevent it. Larger contact area should also help. If you undercut the heel endgrain to leave only 1/8" of contact area around the edge, the longitudinal neck force is focused onto a very small area (but only the last half inch or so at the top is under high pressure, so the rest can be undercut).

With a glued extension, you have a nice big area to transfer the longitudinal neck force to the soundboard in shear.

But in any case, the settling of non-glued necks seems to be a relatively quick process when the guitar is first strung up, and once it's done it remains stable from then on.

I've done one, I epoxied T-nuts in recesses on the bottom of the fretboard - it worked but I had to rework it after a few months when one of the T-Nuts came loose. Notes on the extension tended to not be balanced so I wouldn't recommend my way. If I do it again I'll use something similar to Trevor's design.

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Steve Smith
"Music is what feelings sound like"

Take a look at the bolt-on kit sold by 'Luthier's Cool Tools' (Dave Micheletti). I've used his system very happily on three projects. Stable, secure, easily serviced. Good instructions. One change I made, a floating tenon to key the neck into a stable and positive location without relying on the mounting bolts for location. The change was more to cope with my (lack of) skill than any weakness in design.

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Peter Havriluk

I’ve looked at this system before and was thinking I would do something similar with a tenon as well. Basically a combination of the Cumpiano tenon design with the cool tools bolt down fingerboard design. I’m thinking it would work, but I’m still thinking through the idea.

+1 for the Luthiers Cool Tools kit
It is simple, effective and affordable, and achieves the same thing as Trevor Gore’s bolt-on bolt-down design.
Get the kit and the routing template and you are good to go. As a bonus, I have also adapted the same routing guide to do slotted headstocks.
http://www.luthierscooltools.com/Tools.htm#Bolt-on

In the early tutorials on the Bourgeois style double tenon by John Mayes and Sylvan Wells a separate mortise block glued to the underside of the top was recommended . Even when it was glued to the top brace and butted against the headblock when gluing the top I had more movement than I was comfortable with although it stabilized after 6 months or so.

I think with any double tenon neck it is important to have the extension support/mortise block be integral with The headblock and very tight against the top brace.

I usually leave a slight gap between the end of the extension support and the top brace and then wedge in a sung fitting piece of wood after the box is closed. It is easier to do after the mortise for the fretboard tenon is routed as you can see the gap while wedging in the shim and use some thin CVA to hold it in place. That has helped a lot with the movement but still a little more than a glued extension.

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It's not what you don't know that hurts you, it's what you do know that's wrong.

I haven't seen the aluminum routing template offered for sale by Luthier's Cool Tools, but the layout drawing that comes in the instructions is enough to make one, if a friend with a milling machine is available. That's how I got my template.

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Peter Havriluk

I've played four guitars with bolt-down FB extensions, including experimental guitars by a local builders who's world renown for his classicals, and those that didn't have some mass under the FB, like an extension of the neck (as opposed to just bolted to the top), had weak notes above the body/neck joint. Just sayin'.

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formerly known around here as burbank
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http://www.patfosterguitars.com

Light application of glue, don't clamp down too hard. Stays in place, comes up easily if needed. No extra acrobatics needed. And +1 to the comment on upper register trebles.

Yes, you certainly don’t want to just bolt the fingerboard to the soundboard. Both are too flimsy. But if you fashion an extension of the neck block (inverted L shape) and also glue that to the upper transverse brace then you have a very robust frame. Route into that neck block extension a pocket or channel or some such, and have a corresponding member on the underside of the fingerboard that fits into it. Now you have a solid coupling between the upper part of the body and the end of the fingerboard which can be fastened with a couple of bolts. Much more effective than a thin smear of glue. Easily removed when needed - but I don’t think you will find any deficiency in tone.

This kind of morticed fingerboard joint is solid enough that you can dispense with the usual mortise and tenon neck joint. A simple bolt on butt joint is perfectly sufficient, and much simpler than a dovetail or M&T

I agree with everything Mark just outlined. There is one detail about the support inside the body, under the fingerboard extension, that I want to mention. It might be better to have that tongue splay out a bit to look like a trapezoid. That way, this big block of wood that is glued to the spruce soundboard has edges that are not completely parallel to the spruce’s grain. Some repair folks have seen instances where the string tension makes the whole neck block rotate in a way that causes breaks in the spruce right along those edges. Maybe a trapezoidal shape helps prevent that. I know that John Arnold and others repair old Martins using a trapezoidal block of wood in this spot. Just an idea that I will be implementing in my next build.

Don, that sounds like a very sensible modification